Optical frequency domain imaging (OFDI) techniques, which may be also known as swept source or Fourier-domain optical coherence tomography (OCT) techniques, are OCT procedures which generally use swept laser sources. For example, an optical beam is focused into a tissue, and the echo time delay and amplitude of light reflected from tissue microstructure at different depths are determined by detecting spectrally resolved interference between the tissue sample and a reference as the source laser wavelength is rapidly and repeatedly swept. A Fourier transform of the signal generally forms an image data along the axial line (e.g., an A-line). A-lines are continuously acquired as the imaging beam is laterally scanned across the tissue in one or two directions that are orthogonal to the axial line. The resulting two or three-dimensional data sets can be rendered and viewed in arbitrary orientations for gross screening, and individual high-resolution cross-sections can be displayed at specific locations of interest. This exemplary procedure allows clinicians to view microscopic internal structures of tissue in a living patient, facilitating or enabling a wide range of clinical applications from disease research and diagnosis to intraoperative tissue characterization and image-guided therapy.
The contrast mechanism in the OFDI techniques is generally an optical back reflection originating from spatial reflective-index variation in a sample or tissue. The result may be a so-called intensity image that may indicate the anatomical structure of tissue up to a few millimeters in depth with spatial resolution ranging typically from 2 to 20 μm. While the intensity image can provide a significant amount of morphological information, birefringence in tissues may offer another contrast useful in several applications such as quantifying the collagen content in tissue and evaluating disease involving the birefringence change in tissue. Certain methods and apparatus, so called polarization-sensitive OFDI or OCT, have been utilized. In the conventional methods, the polarization state of probe beam can be alternated between two states in successive axial line (A-line) scans, while the beam is scanned laterally across the sample. Each pair of successive polarization measurements may form a single axial birefringence profile of a sample via the vector analysis. This conventional method utilizes the substantial overlap of the probe beam in the sample between the two A-line scans to avoid speckle-induced errors. Therefore, a compromise can be explored between the accuracy in birefringence measurement and the image acquisition speed. Furthermore, due to the relatively long delay between A-line scans, the conventional method is likely sensitive to a mechanical motion of the sample or catheter.
Exemplary system and method for obtaining polarization sensitive information is described in U.S. Pat. No. 6,208,415. Exemplary OFDI techniques and systems are described in International Application No. PCT/US04/029148. Method and system to determine polarization properties of tissue is described in International Application No. PCT/US05/039374. Using the exemplary OFDI techniques, it may be desirable to implement a balanced detection. However, the balanced detection may complicate the fiber implementation of the polarization sensitivity and polarization diversity because two signal channels that are balanced can have different polarization states.
Accordingly, there is a need to overcome the deficiencies as described herein above.